A quantum well designed to detect infrared radiation (IR) is usually referred to as a quantum well infrared photodetector (QWIP). A candidate for a QWIP is the square quantum well of basic quantum mechanics. When a quantum well is sufficiently deep and narrow, its energy states are quantized (discrete). The potential depth and width of the well may be adjusted so that it holds only two energy states: a ground state near the well bottom, and a first excited state near the well top. A photon striking the well may excite an electron in the ground state to the first excited state, whereby an externally applied electric field sweeps the exited electron out to produce a photocurrent.
A quantum well may be comprised of a first semiconductor sandwiched between two layers of a second semiconductor, where the first and second semiconductors have an energy gap to form the energy well. Quantum wells may be stacked to increase efficiency. FIG. 1a illustrates this in pictorial form, where alternating epitaxial layers of doped GaAs and un-doped Al0.3Ga0.7As are grown on a semi-insulating GaAs substrate by molecular beam epitaxy (MBE). The GaAs epitaxial layers may be doped having an electron donar doping concentration of n=5×1017 cm−3, for example. FIG. 1b illustrates the resulting wells and energy bands in pictorial form, illustrating a photon hv causing an electron to be exited from low energy state 102 to the conduction band, and illustrating electrons 104 in the conduction band contributing to a photocurrent when an electric field is applied.
Only photons having energies corresponding to the energy separation between the two energy states are absorbed, resulting in a photodetector having a relatively sharp absorption spectrum. Designing a quantum well to detect electromagnetic radiation of a particular wavelength becomes a matter of tailoring the potential depth and width of the quantum well to produce two states separated by the desired photon energy. However, QWIPs without light coupling structures do not absorb radiation normal to the surface because the radiation polarization must have an electric field component normal to the superlattice (growth direction) to be absorbed by the confined carriers (e.g., electrons).
FIG. 2 illustrates this in pictorial form, where a single quantum well (202) is illustrated. The vector {right arrow over (p)}z in FIG. 2 has a direction along the growth of the epitaxial layers making up quantum well 202, and denotes the direction of the z axis. As indicated in FIG. 2, the z axis is taken along the growth direction, and the y axis is taken in the plane of the illustration, so that the x axis points out of the illustration. Infrared radiation is indicated by IR in FIG. 2, having a polarization in the x-y plane so that the electric field vector has a zero component in the z direction. The dot product of the electric field vector and the growth direction is zero, and as a result no absorption takes place.